Several literature reports have provided scientific evidence of the antimicrobial activity of cold plasmas operating under atmospheric pressure conditions. The antimicrobial properties of the aforementioned plasmas have been attributed to the presence of ultraviolet radiation, reactive oxygen (ROS), and nitrogen (RNS) species. It is believed that the chemical content of plasmas simulates the natural defense system of multi-cellular organisms that use physiological processes that employ ROS in order to kill foreign microorganisms. The primary biological targets for ROS include DNA, proteins and lipids. The plasma-induced mechanisms currently proposed to interpret bacterial death include, i) electrostatic disruption of cell membranes and ii) membrane or cytoplasmic oxidation, both of which lead to lipid damage, irreversible conformations in the membrane structure, pore formation, and membrane catastrophe. Plasma-generated species may include: superoxides (O2−), hydroxyl radicals (OH−), ozone (O3), hydrogen peroxide (H2O2), nitric oxide (NO), nitrites (NO2), nitrates (NO3) and peroxynitrite (ONOO−). Some of the above mentioned molecules and chemical moieties are found in commercially available liquid disinfectant solutions. Some highly potent radicals (e.g. peroxynitrite), however, are short-lived and therefore would most likely need to be applied immediately after they are produced to ensure rapid disinfection. A strength of plasma usage for decontamination efforts is the mixture of dynamic, antimicrobial ROS and RNS because collectively these species place substantial stress on microbial physiology.
Microorganisms possess enzymatic countermeasures to deal with stressors in their local environments, including superoxide dismutase, catalase, peroxidase, etc., but after a critical plasma application threshold, these enzymes cannot clear the oxidative stress, and the microbes ultimately succumb to the applied plasma exposure. The combination of rapid, potent antimicrobial radicals with more stable species that still possess antimicrobial efficacy (nitrates) makes it very difficult for microbes to develop resistance to a synergistic plasma effect.
Application of atmospheric cold plasmas upon pathogenic bacteria such as E. coli and S. aureus suspended in water or other liquid biological media has been successfully demonstrated. However, the application of atmospheric plasmas in complex, real life settings, such as nosocomial environments, would require direct contact of the plasma with dry contaminated surfaces.
Sterilization of medical equipment using radicals produced by oxygen/water vapor RF plasma has been reported, but the proposed process involved the use of vacuum equipment rendering it inapplicable for large area disinfection. The most distinctive chemical feature of the cold plasma processes presented in this paper is their ability to cause multiple chemical reactions at atmospheric pressure and room temperature. Another paper presented a 1.5 log10 (CFU/cm2) reduction for S. epidermidis and 1.8-2.0 log10 (CFU/cm2) for P. aeruginosa residing on polycarbonate discs after 30 s-600 s of treatment with air surface barrier discharge.